Optical lithography has been one of the principal driving forces behind the continual improvements in the size and performance of the integrated circuit (IC) since its inception. Feature resolution down to 0.30 xcexcm is now routine using the 365 nm mercury (Hg) i-line wavelength and optical projection tools operating at numerical apertures above 0.55 with aberration levels below 0.05xcex RMS OPD. The industry is at a point where resolution is limited for current optical lithographic technologies. In order to extend capabilities toward sub-0.25 xcexcm, modifications in source wavelength, optics, illumination, masking, and process technology are required and are getting very much attention.
However, as devices get smaller, the photomask pattern becomes finer. Fine patterns diffract light and thus detract from imaging the photomask onto the surface of a wafer. FIG. 1a shows what happens when a photomask with a fine pattern 6 having a high frequency (pitch 2d is about several microns), is illuminated through a projection lens system 7. The fine pattern 6 is illuminated along a direction perpendicular to the surface thereof and it diffracts the light that passes through the mask 6. Diffraction rays 3-5 caused by the pattern include a zero-th order diffraction ray 4 directed in the same direction as the direction of advancement of the input ray, and higher order diffraction rays such as positive and negative first order diffraction rays 3, 5, for example, directed in directions different from the input ray. Among these diffraction rays, those of particular diffraction orders such as, for example the zero-th order diffraction ray and positive and negative first order diffraction rays 3, 5, are incident on a pupil 1 of the projection lens system 7. Then, after passing through the pupil 1, these rays are directed to an image plane of the projection lens system, whereby an image of the fine pattern 6 is formed on the image plane. In this type of image formation, the ray components which are contributable to the contrast of the image are higher order diffraction rays. If the frequency of a fine pattern increases, it raises a problem that an optical system does not receive higher order diffraction rays. Therefore, the contrast of the image degrades and, ultimately, the imaging itself becomes unattainable.
As will be shown below, some solutions to this problem rely upon shaping the rays of light impinging the photomask in order to provide off-axis illumination to compensate for the lost contrast due to diffraction. These techniques rely upon optical systems for shaping the rays that illuminate the photomask.
In considering potential strategies for sub-0.25 xcexcm lithography, the identification of purely optical issues is difficult. Historically, the Rayleigh criteria for resolution (R) and depth of focus (DOF) has been utilized to evaluate the performance of a given technology:
R=k1xcex/NA 
DOF=+/xe2x88x92k2xcex/NA2 
where k1 and k2 are process dependent factors, xcex is wavelength, and NA is numerical aperture. As wavelength is decreased and numerical aperture is increased, resolution capability improves. Considered along with the wavelength-linear and NA-quadratic loss in focal depth, reasonable estimates can be made for system performance. Innovations in lithography systems, materials and processes that are capable of producing improvements in resolution, focal depth, field size, and process performance are those that are considered most practical.
The Hg lamp is a source well suited for photolithography and is relied on almost entirely for production of radiation in the 350-450 nm range. Excimer lasers using argon fluoride (ArF) and krypton fluoride (KrF), which produce radiation at 193 nm and 248 nm, respectively, are also used. DUV lithography at 248 nm is now being implemented into manufacturing operations and may be capable of resolution to 0.18 xcexcm.
The control of the relative size of the illumination system numerical aperture has historically been used to optimize the performance of a lithographic projection tool. Control of this NA with respect to the projection systems objective lens NA allows for modification of spatial coherence at the mask plane, commonly referred to partial coherence. This is accomplished through specification of the condenser lens pupil size with respect to the projection lens pupil in a Kxc3x6hler illumination system. Essentially, this allows for manipulation of the optical processing of diffraction information. Optimization of the partial coherence of a projection imaging system is conventionally accomplished using full circular illuminator apertures. By controlling the distribution of diffraction information in the objective lens with the illuminator pupil size, maximum image modulation can be obtained. Illumination systems can be further refined by considering variations to full circular illumination apertures. A system where illumination is obliquely incident on the mask at an angle so that the zero-th and first diffraction orders are distributed on alternative sides of the optical axis may allow for improvements. Such an approach is generally referred to as off-axis illumination. The resulting two diffraction orders can be sufficient for imaging. The minimum pitch resolution possible for this oblique condition of partially coherent illumination is 0.5xcex/NA, one half that possible for conventional illumination. This is accomplished by limiting illumination to two narrow beams, distributed at selected angles. The illumination angle is chosen uniquely for a given wavelength (xcex), numerical aperture (NA), and feature pitch (d) and can be calculated for dense features as sinxe2x88x921 (0.5xcex/d) for NA=0.5xcex/d. The most significant impact of off axis illumination is realized when considering focal depth. In this case, the zero-th and 1st diffraction orders travel an identical path length regardless of the defocus amount. The consequence is a depth of focus that is effectively infinite.
In practice, limiting illumination to allow for one narrow beam or pair of beams leads to zero intensity. Also, imaging is limited to features oriented along one direction in an x-y plane. To overcome this, an annular or ring aperture has been employed that delivers illumination at angles needed with a finite ring width to allow for some finite intensity. The resulting focal depth is less than that for the ideal case, but improvement over a full circular aperture can be achieved. For most integrated circuit application, features are limited to horizontal and vertical orientation, and a four-zone configuration may be more suitable. Here, zones are at diagonal positions oriented 45 degrees to horizontal and vertical mask features. Each beam is off-axis to all mask features, and minimal image degradation exists. Either the annular or the four-zone off-axis system can be optimized for a specific feature size, which would provide non-optimal illumination for all others. For features other than those that are targeted and optimized for, higher frequency components do not overlap, and additional spatial frequency artifacts are introduced. This can lead to a possible degradation of imaging performance.
When considering dense features (1:1 to 1:3 line to space duty ratio), modulation and focal depth improvement can be realized through proper choice of illumination configuration and angle. For true isolated features, however, discrete diffraction orders would not exist; instead a continuous diffraction pattern is produced. Convolving such a frequency representation with either illumination zones or annular rings would result in diffraction information distributed over a range of angles. Truly isolated line performance is, therefore, not improved with off-axis illumination. When features are not completely isolated but have low density ( greater than 1:3 line to space duty ratio), the condition for optimum illumination will not be optimal for more dense features. Furthermore, the use of off-axis illumination is generally not required for the large pitch values that correspond to low density geometry. As dense and mostly isolated features are considered together in a field, it follows that the impact of off-axis illumination on these features will differ, and a large disparity in dense to isolated feature performance can result.
One approach to generate off-axis illumination is to incorporate a metal aperture plate filter into the fly eye lens assembly of the projection system illuminator providing oblique illumination. A pattern on such a metal plate would have four quadruple openings (zones) with sizing and spacing set to allow diffraction order overlap for specific geometry sizing and duty ratio on the photomask, as disclosed in JP patent Laid-Open (KOKAI) Publication No.4-267515. Such an approach results in a significant loss in intensity available to the mask, lowering throughput and making the approach less than desirable. Additionally, the four circular openings need to be designed specifically for certain mask geometry and pitch and would not improve the performance of other geometry sizes and spacings. Large levels of mask biasing or mask optical proximity correction (OPC), where mask features are pre-distorted to produce desired image characteristics, would be required to allow for use of this approach with a variety of features. Filtering, by limiting its effective area, reduces the effect of the fly eye diffuser on maximizing illumination uniformity. Illumination uniformity may be degraded. This approach also limits the illumination profile to one having holes in a metal plate. That is, the masking metal must remain contiguous. The previous work in this area describes such methods using either two or four openings in the aperture plate: EP0500393, U.S. Pat. Nos. 5,305,054, 5,673,103, 5,638,211, EP0496891, EP0486316, U.S. Pat. No. 37,9252.
Another approach to off-axis illumination using the four-zone configuration, which is disclosed in U.S. Pat. No. 5,627,625, is to divide the illumination field of the projection system into beams that can be shaped to distribute off-axis illumination to the photomask. By incorporating the ability to shape off-axis illumination, throughput and flexibility of the exposure source is maintained. Additionally, this approach allows for illumination that combines off-axis and on-axis (conventional) characteristics. By doing so, the improvement to dense features that are targeted with off-axis illumination is less significant than straight off-axis illumination. The performance of less dense features, however, is more optimal because of the more preferred on-axis illumination for these features. The result is a reduction in the optical proximity effect between dense and isolated features. Optimization is less dependent on feature geometry and more universal illumination conditions can be selected.
A problem with this divided illumination approach is that it requires reconfiguration of the illumination system of a projection tool, a task that is not practical on existing tools or systems designed with other illumination systems. Additionally, the use of divided beam illumination limits the fine control of beam shape, size, and position to that which is possible with optical components utilized in the system. Variations in shape, size, position, number of beams, maximum aperture size, or other feature or lens specific variations to the illumination intensity profile become difficult without significant mechanical modifications. Some variations may not be practical or possible with this approach. This has significantly limited the acceptance or use of this approach in most integrated circuit fabrication operations.
A shaped illumination approach is described that allows for off-axis illumination of a photomask in a projection exposure tool. It is necessary to control both the off-axis and the on-axis character of the illumination for instance so that dense line performance can be improved and more isolated line performance can be maintained, i.e., optical proximity effect (hereinafter xe2x80x9cOPExe2x80x9d) is kept to a minimum. Minimal OPE correction is desired to reduce mask complexity. There is also a desire for a flexible technique that can be incorporated into most existing or future generation projection exposure tools with a minimum amount of illumination system retrofitting. It is important that such an improvement be easily removed to allow a return to original operation conditions since it is expected that a given projection exposure system would be used for a variety of applications. Such an approach can lead to optimizing illumination conditions, which can be incorporated into an exposure system as a more permanent condition. The invention also provides an improvement that allows for fine adjustment or modification to accommodate mask-specific or lens-specific characteristics is important as resolution and focal depth requirement are pushed beyond the capability of conventional optical lithography. Maintaining illumination throughput is also critical, as any loss in intensity will result in increased exposure time requirements.
The invention provides such a solution. It modifies existing illumination system by adding a masking aperture in the illumination pupil plane, fabricated as an optical component reticle, patterned and dithered into a large number of elements to allow for control of the projected light distribution at the mask plane and inserted at the condenser lens pupil plane. This masking aperture comprises a translucent substrate and a masking film. The distribution of the intensity through the masking aperture in the illumination pupil plane, is determined to provide optimized illumination. The illumination region or regions exhibit varying intensity, which is accomplished by creating a half-tone pattern via pixelation of the masking film, thereby allowing for maximum variation in illumination beyond the simple binary (clear or opaque) possible with earlier pupil plane filtering approaches.
More specifically, the invention includes an aperture mask for an illumination system to provide controlled on-axis and off-axis illumination. The aperture mask acts as a diffraction element. The aperture mask is divided into an array of elements and each element contains an array of pixels. Each of the elements is constructed with a matrix of pixels. In the preferred embodiment the array of pixels is 8xc3x978. The number of elements in the illumination array are generally larger than 3xc3x973 and array sizes of 21xc3x9721 and 51xc3x9751 are an example, which correspond to 441 and 2601 elements respectively. The elements are patterned in accordance with a selected wavelength of incident light to diffract the incident light into an illumination pattern for illuminating a photomask.
The intensity is modulated by the intensity state of pixels within each element. The highest intensity element has all pixels clear or maximum intensity. Light of suitable wavelength passes through without attenuation. An element with 64 pixels having dark or minimum intensity attenuates or blocks all light. Elements of intensity between none (0%) and all (100%) are created by the state of the pixels in a given element. Random patterns and other patterns between elements may produce artifacts similar to moire patterns. Such artifacts are undesired. I discovered that a dithered pattern using position dependent thresholds produced illumination patterns that had little or no artifacts.
I also discovered that traditional, optical shaping systems such as beam-splitters or diffractive optical elements and my diffraction shaping system can each be improved by adding an illumination aperture. A square illumination aperture shows maximum improvement. It is located at or near the pupil of the illumination system. An aperture with a large, central square opening and an opaque border is inserted at the condenser lens pupil proximate to the fly""s eye lens. It can also be designed into the masking aperture. The resulting illumination pattern fills the corners and squares the edges of the illumination pupil and limits the non-optimal frequency spreading character along the x and y axes while optimizing the off-axis illumination angles. The square illumination aperture is especially helpful for imaging features that are oriented along x and y directions in the mask plane. The use of a central obscuration (square and also round shaped) applied in the same location can similarly be achieved and can lead to performance improvements described hereinafter. Furthermore, any combination of off-axis illumination with a square pupil or obscuration has potential to improve performance for geometry oriented in the x-y direction. This can include, but is not limited to, round zones, elliptical zones, square zones, and annular slots (that is an annular ring masked off on x and y axis to form arc-shaped zones).